1. Field of the Invention
The present invention relates to a display apparatus using a cholesteric liquid crystal with an image memory function that the apparatus is capable of retaining a display state without requiring electric power (noted as “power” hereinafter), and particularly to a background color forming method for a cholesteric liquid crystal display (LCD) element.
2. Description of the Related Art
Recent years have been witnessing a use of a cholesteric LCD element capable of retaining a display state without requiring the electricity, that is, capable of performing an image memory display, as a segment type display apparatus, et cetera. The cholesteric liquid crystal noted here is precisely defined as a selective reflection type cholesteric liquid crystal which has two stable states, i.e., an interference reflection state (i.e., a color reflection) for reflecting only a specific color and a transparent state (i.e., black) for transmitting light and which has a characteristic, namely a bistability, of being capable of retaining respective display states without requiring a power after being electrically driven. The cholesteric liquid crystal has a spiral molecular structure and a property of selective reflection reflecting only a light of a wavelength corresponding to the pitch of the spiral in the interference reflection state. Therefore, a use of a cholesteric liquid crystal having a spiral of a pitch corresponding to a wavelength of a color to be desirably reflected enables a desired color display.
Meanwhile, the cholesteric liquid crystal also has a property of mutual transition between the interference reflection state and transparent state which is caused by an external pressure and/or a thermal environment. Because of this, there is a problem of being very difficult to revert back to an initial state if an external action results in a change of states in a part in which a counter electrode does not exist, such as an outer circumferential part of a segment and in between electrode wirings.
Therefore, in an LCD apparatus, such as a conventional segment type display apparatus, a background color has been limited to black even though the segment part is a color display as shown in FIG. 1. In the technical fields, such as clock, marker display, et cetera, putting emphasis on an industrial design, however, a colorful background is highly desired as shown in FIG. 2.
The following is a description of reason for a background color being limited to black in a conventional display element using a conventional cholesteric liquid crystal by referring to FIGS. 3 and 4.
FIG. 3 is a diagram of a conventional cholesteric liquid crystal display element 10 when looking at it from the display surface side. FIG. 3 shows an example of a segment display of the number “35” in color with black as the background. Also shown are segment lead wires 12 for applying a voltage to each segment electrode 11.
FIG. 4 shows a cross-section of the conventional cholesteric LCD element 10.
What are shown here are, from the display surface side, a layer of segment electrodes 11 and light-shield film 13, a liquid crystal unit (i.e., a cell) 14 filled with a cholesteric liquid crystal, a light absorption layer 15, a common electrode (i.e., a flat type electrode) 16 and a glass substrate 17, while segment lead wires 12 are omitted. The segment electrodes 11 are transparent, while the light-shield film 13 and light absorption layer 15 are black.
In the conventional cholesteric LCD element 10, a part which is a background part other than a segment part where there is no counter electrode pair part such as the segment electrode 11 and common electrode 16 is made black by forming the light-shield film 13 shielding a cell 14 of which the state can possibly be changed by an external action so as to make a reflection color of a segment as an On display and a black color of a transparent state, which is the same as the background color, as an Off display. This accordingly limits the background color to black.
It is of course easily possible to conceive a method for making a colorful background by coloring the light-shield film 13 to form a background color, and reversibly displaying negative (On) and positive (Off) displays; which, however, requires a segment reflection color to be exactly identical with a background color in order to bury the display color completely in the background. The reflection color of a segment is an interference reflection color of a liquid crystal expressing a special shade dependent on a view angle and it is therefore very difficult to make it identical with the background color which is colored with a pigment or dye.
Meanwhile, it is possible to form a background color by using a liquid crystal by comprising a background-use electrode; an inter-electrode space of tens micrometers needs to be formed for insulating a border with a display pattern electrode, however. Because of this space, it is not possible to bury the display color completely in the background.
As an example, each of the following reference patent documents 1 and 2 notes an LCD element forming a background color by comprising a background-use electrode; either of them, however, has not been able to bury the display color completely in the background since there is a gap, although it may be very small, between the electrode for the display pattern and that for the background.
As a configuration colorizing a background, the following reference patent document 3 notes a display element filling display cell with a liquid crystal, in which a transition from a cholesteric phase to a nematic phase is caused by applying an electrical field, added with a multi-color dye, and controlling a hue by applying an electrical field, thereby performing a display. This display element, however, has no image memory function and therefore a constant application of the electrical field is required for maintaining a display state.
Patent document 1: Japanese Published Patent Application No. H11-337672
Patent document 2: Japanese Published Patent Application No. 2002-229051
Patent document 3: Japanese Published Patent Application No. S53-35564